1. Field of the Invention
The present invention relates generally to optical transmission, and particularly to an optical transmission system with reduced optical signal pulse width of a modulated output signal.
2. Technical Background
Until recently, most fiber optic communications systems employed Nonreturn-to-Zero (NRZ) modulation in which each logical bit, at a logical state, or pulse was transmitted as a pulse whose full pulse width was equal to the full bit period, T=1/B, where B is the bit rate at which pulses are transmitted. More recently, it has been shown that Return-to-Zero or RZ signaling provides better performance in terms of reduced degradation of dense wavelength division multiplexed (DWDM) system performance due to fiber non-linearities such as four wave mixing (FWM) and cross-phase modulation (XPM).
RZ modulation can be implemented in a variety of known different binary signaling formats, which all have significantly different optical transmission spectra. Of these formats, unipolar RZ or single-phase RZ (RZ-SP) is the most straight forward to implement in optical fiber transmission systems, although what has become known as optical carrier-suppressed RZ (CS-RZ) and duobinary have also been implemented as novel modulation techniques that resist fiber nonlinear impairments on carefully engineered dispersion-managed transmission links.
CSRZ optical signals have the feature of presenting bits that are phase-shifted relative to neighboring bits such that on the average all the phases cancel each other out for a net phase of zero. For example, a CSRZ optical data stream may consist of a plurality of CSRZ output pulses where half of the pulses have an alternating phase relationship with the other half of the pulses, which leads to carrier suppression. Because of this phase inversion between adjacent bit periods that reduces inter-bit interference, CSRZ signals show increased tolerance to dispersion and to nonlinear penalties such are intrachannel four-wave mixing (FWM).
Furthermore, CSRZ signals subject the sensitivity of the receiver to little degradation at high power. CSRZ signals are also less sensitive to self-phase modulation (SPM), in addition to the other non-linearities already mentioned. It is also known that the duobinary format and CSRZ both have two optical electric field phase states +/−E that represent logical ONEs or MARKs, that both have the same optical power level P. When these signals are detected by the usual PIN photodiodes, which respond to the optical power, no discernible difference between the two states results, so that detection is quite simple at the receiver.
High capacity fiber-optic transmission systems such as dense wavelength-division-multiplexed (WDM) and/or time-division multiplexed (TDM) systems with many closely spaced wavelength and/or time channels modulating at high bit or data rates are required to meet the growing demand of Internet traffic bandwidth and other telecommunication systems. High data rate transmission using bit rates greater than 40 Gb/s is currently at the cutting edge of technology in fiber optic communication systems. To generate signals for a 40 Gb/s WDM system, high-speed and costly electronics have to be used. Due to the limitations of current modulators, bit rates of 80 or 160 Gb/s are usually obtained through optical time-division multiplexing (OTDM) of multiple independently modulated 40 Gb/s signals, all at the same carrier frequency but interleaved in time. In order to accomplish OTDM, the pulses of the 40 Gb/s signals must be compressed, in separate conventional pulse compressors, to an ultra-short duty cycle to avoid overlapping during multiplexing. Unfortunately, the process of pulse compression has so far been quite expensive, involving either a serial concatenation of modulators or a combination of chirped pulses with dispersion decreasing fiber. Recently, there have been some novel ideas for soliton generation to generate variable duty cycle ultra-short pulses using a single NRZ Mach-Zehnder modulator but these single modulators were not multiplexed.
Therefore there is a need to implement novel modulation techniques for converting a unipolar voltage data stream into a CSRZ optical data stream in an improved and simple optical modulator structure that can be multiplexed to mitigate nonlinear transmission impairments in fiber-optic communication systems for transmitting ultra-short pulses.